To reduce the amount of queries to the driver, we can cache the
results of glGetString. On Mac, we need to invalidate this cache
on GPU switch.

Bug: chromium:1173672
Change-Id: I039172068aec35034a87881a8804f52c080ce4ce
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2676882
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>